This invention is directed to an improved method of controlling low speed and “creeping” performance of machines that use hydrostatic transmissions in propel applications. More specifically, the present invention is directed to a method of improved control for industrial trucks and forklift trucks that use hydrostatic transmissions to transmit power from an internal combustion engine to the wheels of the machine.
Forklift trucks are used for purposes such as the loading and unloading of transportation vehicles, the stacking and transporting of goods through warehouses, and for other well known purposes. In the operation of a fork lift truck, the ability of an operator to control precise movements of the truck is critical. This precise or creeping movement is especially important when positioning the fork into a load and when positioning a load into location. As such operators expect machines to allow accurate positioning of a payload with a precision of less than a centimeter.
This capability of controlling creeping performance is currently available in battery powered industrial trucks. This quality of controlling creeping performance may also be available on some models of internal combustion engine trucks that use gear based transmissions and torque converters. Unfortunately, the prior art does not teach a satisfactory method to achieve the control of creeping performance and/or quality in internal combustion engine driven industrial trucks using hydrostatic propel transmissions.
One of the reasons pure displacement “mode” transmissions do not perform adequately in forklift applications is that the engines are relatively low power and have flat torque curves. Thus, the traditional displacement mode transmission will cause engine stall at displacements above +/−5% from neutral.
The control system of a hydrostatic industrial truck often consists of an operator input device such as a foot pedal, an electronic controller, and an electrical control such as an “Electronic Displacement Control” on at least one hydrostatic power unit. Several control methods are known in the prior art for the control of hydrostatic transmissions. More specifically, hydrostatic transmissions are sometimes controlled in a displacement control mode. In displacement control, the displacement of a hydrostatic transmission is a direct and normally linear function of the operator's input command. At a given engine speed this will cause the speed of the vehicle to be directly proportional to the foot pedal input command position. Speed, torque, and power control schemes are also well known in the prior art.
Thus, a problem exists in the art in that when a hydrostatic transmission is used in a device such as a forklift that needs low speed and creeping performance, the transmission does not have a means to provide for this low speed performance and at the same time provide a second control scheme for other working conditions. Consequently, there is a need in the art for a control system that is able to detect the desired operating mode of a forklift and cause the hydrostatic transmission to perform in that mode so that the correct control scheme is being used by the forklift for the task at hand.
Thus, it is a primary object of the present invention to provide a control system for a hydromechanical transmission that will place the transmission in a first mode for low speed applications and a second mode for high speed applications.
Another object of the present invention is to provide an operating system for a hydromechanical transmission that allows a forklift to operate in a first mode that is a displacement control mode and also in a second mode that is a different control scheme.
Yet another object of the present invention is to provide a forklift or truck that may be operated in multiple modes using only a single foot pedal.
Another object of the present invention is to provide for a device that will allow an operator to select the control scheme of a forklift.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.